ABSTRACT Considerable advances in chronobiology have been made through controlled laboratory studies, but distinct temporal rhythms can emerge under natural environmental conditions. Lab-reared Nematostella vectensis sea anemones exhibit circadian behavioral and physiological rhythms . Given that these anemones inhabit shallow estuarine environments subject to tidal inputs, it was unclear whether circadian rhythmicity would persist following entrainment in natural conditions, or whether circatidal periodicity would predominate. Nematostella were conditioned within a marsh environment, where they experienced strong daily temperature cycles as well as brief tidal flooding around the full and new moons. Upon retrieval, anemones exhibited strong circadian (∼24 h) activity rhythms under a light–dark cycle or continuous darkness, but reduced circadian rhythmicity under continuous light. However, some individuals in each light condition showed circadian rhythmicity, and a few individuals showed circatidal rhythmicity. Consistent with the behavioral studies, a large number of transcripts (1640) exhibited diurnal rhythmicity compared with very few (64) with semidiurnal rhythmicity. Diurnal transcripts included core circadian regulators, and 101 of 434 (23%) genes that were previously found to be upregulated by exposure to ultraviolet radiation. Together, these behavioral and transcriptional studies show that circadian rhythmicity predominates and suggest that solar radiation drives physiological cycles in this sediment-dwelling subtidal animal.

ABSTRACT The increased incidence of solar ultraviolet B (UVB) radiation has been proposed as an environmental stressor, which may help to explain the enigmatic decline of amphibian populations worldwide. Despite growing knowledge regarding the UV-induced biological effects in several amphibian models, little is known about the efficacy of DNA repair pathways. In addition, little attention has been given to the interplay between these molecular mechanisms with other physiological strategies that avoid the damage induced by sunlight. Here, DNA lesions induced by environmental doses of solar UVB and UVA radiation were detected in genomic DNA samples of treefrog tadpoles ( Hypsiboas pulchellus ) and their DNA repair activity was evaluated. These data were complemented by monitoring the induction of apoptosis in blood cells and tadpole survival. Furthermore, the tadpoles’ ability to perceive and escape from UV wavelengths was evaluated as an additional strategy of photoprotection. The results show that tadpoles are very sensitive to UVB light, which could be explained by the slow DNA repair rates for both cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6,4) pyrimidone photoproducts (6,4PPs). However, they were resistant to UVA, probably as a result of the activation of photolyases during UVA irradiation. Surprisingly, a sensory mechanism that triggers their escape from UVB and UVA light avoids the generation of DNA damage and helps to maintain the genomic integrity. This work demonstrates the genotoxic impact of both UVB and UVA radiation on tadpoles and emphasizes the importance of the interplay between molecular and sensory mechanisms to minimize the damage caused by sunlight.

SUMMARY This study evaluates in vitro the effects of UVB irradiation on three cellular compartments of a shallow water coral species. Coral tissues were dissociated by Ca 2+ -Mg 2+ -free artificial seawater. Cell suspensions were divided into the major cellular compartments (animal cells, algal cells, holobiont entities) by sucrose gradient and then by detergent treatments. Cell fractions were irradiated by UVB lamp (4.05, 8.1 and 12.2 kJ m –2 ) and subjected to the comet assay. UVB radiation, at levels that induced a moderate DNA breakage to the non-symbiotic coral and algal cell compartments, caused dramatic increase in DNA breakage to the holobiont entities. After a 1 h repair period, DNA breakage levels in the algal and animal cell fractions were augmented as compared with a reduction in DNA breakage in the holobiont fraction. This discordancy in DNA breakage between the three cellular compartments reveals that the holobiont cell fraction is more vulnerable to increased natural UV irradiation and associated anthropogenic genotoxic impacts, providing another possible explanation for recent increase in worldwide coral bleaching events.